The foregoing data link control devices provide advanced levels of connectivity and functionality between computer systems and communication networks; particularly, networks operating under Integrated Services Digital Network (ISDN) protocols as defined in existing international standards (refer to Integrated Services Digital Network (ISDN), Recommendations of the Series I, CCITT VIIIth Plenary Assembly, Geneva 1985, and particularly to Volume III, Fascicle III.5 of that proceeding (hereafter cited as ISDN Fascicle 111.5).
Such networks are designed to be able to sustain transmission of digital data signals, and voice grade digital telephone signals, at bit rates beyond capabilities of many of today's analog telephone networks. Data links on such networks generally sustain groups of "nB+D" serial data communication channels (refer to ISDN Fascicle III.5, recommendation 1.412, section 3), and adapter (link control) devices operating between such links and computer systems or devices (host systems) must support communications relative to such channels. The devices described in the cross-referenced patent applications above, also perform additional functions mentioned above (insertion/deletion of characters, check functions, etc) to relieve the host system of corresponding functional burdens.
The IDLC device described in cross-referenced applications 1-3 is designed to sustain communication relative to a "30B+D" channel group (30 B channels and one D channel). The IIM device described in application 4 above is designed to sustain communication relative to either one or two groups of "2B+D" channels (two B channels and one D channel in each group). The IIM also can perform certain functions relative to telephone networks which were not contemplated for the IDLC. These functions and associated circuits and applications are subjects of the present invention.
What is considered innovational presently is a recognition that a need exists for "seamless" integration of certain telephone services into today's desktop and mid-size computer systems, and that a "serendipitous" focal point of such integration is in the communications adaption path between the system and data communication links. The convenience or serendipity of this choice of focus will be understood as this disclosure proceeds.
The enhanced IIM device described herein provides a basis for seamless integration of many telephone related services into computer systems. It contains a telephone attachment port, data link ports and circuits for transferring data signals selectively either between memory in a host computer system and any channel (B, or D) on its data link ports or "directly" between the telephone port and one of the B channels on a data link port. To a user of a host computer system, a telephone attached to the device's telephone port, appears to be an integral extension of the system, while imposing relatively small processing and I/O contention burdens on central resources of the system shared by other devices (memory, bus, system CPU, etc.).
A problem in attaching telephones to computer systems, is that voice grade telephone signals associated with real-time telephone conversations must not be degraded by excessive or unpredictable delays. Consequently, it is recognized presently that such signals must not be routed through computer system resources which are shared by other devices (e.g. system I/O bus and memory); since contention for such resources could introduce delays which may degrade transferred voice signals in an unacceptable manner. As adapted presently, the IIM provides isochronal handling of real time voice signals which meets this requirement.